Single-ply membrane sheet roofing systems are very well recognized and widely in use as both new and renovated exterior roof surfaces for a multiplicity of building structures having generally flat roof decks. Such deck sheets today are custom prefabricated in the factory by Duro-Last, Inc., applicant's assignee, to the exact dimensions of the building roof and furnished, with weight considerations in mind, in rolled transportable sections of up to 2500 square feet to the roofer on site. Other single-ply roofs are largely worker-constructed at the site and bonded by the roofer on the site. Presently, in the case of Duro-Last, Inc. up to eighty-five percent of the field seams can be completed in the factory under ideal factory conditions, eliminating waste, saving labor, and preventing leaks. A number of such roofing systems are utilized for large footprint roofs, such as factories, administrative buildings, schools, and office buildings, for example. The present invention is concerned with improvements which are particularly suited to the more difficult-to-install systems where the number of necessary fasteners utilized, from the standpoint of labor cost, must be kept to a minimum, while still effectively performing their function.